nimble/phy: Allow extra t_ifs margin on receive

This is only for debugging purposes as some controllers violate t_ifs
and we won't be now able to maintain connection with such hw.

X-Original-Commit: a0434c3fd666ea049318b866ca625ab483c34716

nimble/drivers/nrf52/src/ble_phy.c

@@ -484,6 +484,17 @@ ble_phy_wfr_enable(int txrx, uint8_t tx_phy_mode, uint32_t wfr_usecs)
         end_time += g_ble_phy_t_txenddelay[tx_phy_mode];
         /* Wait a bit longer due to allowed active clock accuracy */
         end_time += 2;
+#if MYNEWT_VAL(BLE_PHY_CODED_RX_IFS_EXTRA_MARGIN) > 0
+        if ((phy == BLE_PHY_MODE_CODED_125KBPS) ||
+                                    (phy == BLE_PHY_MODE_CODED_500KBPS)) {
+            /*
+             * Some controllers exceed T_IFS when transmitting on coded phy
+             * so let's wait a bit longer to be able to talk to them if this
+             * workaround is enabled.
+             */
+            end_time += MYNEWT_VAL(BLE_PHY_CODED_RX_IFS_EXTRA_MARGIN);
+        }
+#endif
     } else {
         /*
          * RX shall start no later than wfr_usecs after RX enabled.

nimble/drivers/nrf52/syscfg.yml

@@ -19,6 +19,16 @@
 # Package: hw/drivers/nimble/nrf52
 
 syscfg.defs:
+    BLE_PHY_CODED_RX_IFS_EXTRA_MARGIN:
+        description: >
+            This defines additional margin for T_IFS tolerance while in
+            RX on coded phy to allow maintaining connections with some
+            controllers that exceed proper T_IFS (150 usecs) by more
+            than allowed 2 usecs.
+            This value shall be only used for debugging purposes. It is
+            strongly recommended to keep this settings at default value
+            to ensure compliance with specification.
+        value: 0
     BLE_PHY_DBG_TIME_TXRXEN_READY_PIN:
         description: >
             When set to proper GPIO pin number, this pin will be set